1. Field of the Invention
The present invention relates to a filter circuit. More specifically, the present invention relates to a filter circuit employing a surface acoustic wave device for use in a video intermediate frequency circuit of a television receiver, for example.
2. Description of the Prior Art
A video intermediate frequency filter using a surface acoustic wave device in a television receiver, for example, has been proposed and put into practical use. Application of a surface acoustic wave device as a video intermediate frequency filter, for example, is disclosed in U.S. Pat. No. 3,582,838, issued June 1, 1971 to Adrian J. DeVries and entitled "SURFACE WAVE DEVICES." On the other hand, it is required that a video intermediate frequency filter have an ample amount of attenuation at a predetermined frequency such as the sound carrier frequency of the adjacent channel to eliminate interference. In utilizing a surface acoustic wave filter as a video intermediate frequency filter, for example, as in the case of the patent referred to above, a surface acoustic wave filter is designed to meet the above described requirement in full consideration of the pattern and the like of the interdigital electrode of the filter. However, in actuality a sufficient degree of attenuation amount can not often be attained due to reflection loss, variation in the characteristic of the products and so on. Furthermore, since a surface acoustic wave filter pertains to a category of a filter of a so-called concentrated constant type, a surface acoustic wave filter is very often used in combination with an integrated circuit as a matter of practice; however, it is extremely difficult to implement an integrated circuit so as to exhibit a filter characteristic and accordingly it is necesary to attain a desired frequency characteristic with a surface acoustic wave filter in employing such filter in combination with an integrated circuit. In other words, in employing a surface acoustic wave filter in combination with an integrated circuit, a portion for inserting an interstage tuning element is often unavailable. Therefore, one might think of implementing a filter circuit having a large degree of attenuation pole of a sufficiently large attenuation by employing a given auxiliary element in association with a surface acoustic wave device.
FIG. 1 is a schematic diagram showing one example of a filter circuit which may be considered in the light of the foregoing background. The FIG. 1 filter circuit comprises a surface acoustic wave device 1. The surface acoustic wave device 1 comprises an input transducer 2 and an output transducer 3, each including an interdigital electrode. The input transducer 2 is connected to an input signal source 6. A resistor 7 denotes a resistive component of the signal source 6. The output transducer 3 is connected through a load resistor 8 to the subsequent circuit 9. A piezoelectric resonator 10 is connected in parallel with the input transducer 2. The piezoelectric resonator 10 may be connected in parallel with the output transducer 3, as shown by the dotted line in FIG. 1. The piezoelectric resonator 10 is used as a trap device. Accordingly, a component of the frequency equal to the resonance frequency of the piezoelectric resonator 10 out of the input signal obtained from the input signal source 6 is trapped through the resonator 10 and will not be passed to the load resistor 8 and the subsequent circuit 9. Thus, although the FIG. 1 circuit is entirely different in principle from the present invention, a sufficient amount of attentuation can be attained with respect to a given frequency.
However, the FIG. 1 circuit involves the following problem to be solved. More specifically, assuming that the impedance of the surface acoustic wave filter 1 and the impedance of the piezoelectric resonator 10 in the passband of the filter are Zs and Zx, respectively, where the impedance Zx is solely dependent on the capacitance component of the resonator 10, and these impedances Zs and Zx have the relation Zs Zx, then the insertion loss of the filter circuit increases and the signal level at the passband decreases. The reason is that since the impedance Zx of the resonator 10 is small, the resonator 10 comes to short-circuit the transducer of the surface acoustic wave filter 1, and accordingly, although a sufficient amount of attentuation is attained at a given frequency corresponding to the resonance frequency of the resonator 10, say at adjacent sound carrier frequency, the above described insertion loss becomes large. Conversely, assuming that the relation of these impedances Zs and Zx is Zs Zx, very little influence is exerted upon the passband; however, the extent of an improvement of the amount of attenuation at the given frequency would become smaller. Accordingly, the mere connection of a piezoelectric resonator as a trap device in parallel with a transducer, as shown in FIG. 1, does not bring about a satisfactory result.